1. Field of the Invention
The present invention relates to rotating media storage devices (RMSDs). More particularly, the present invention relates to an RMSD that performs servo synchronization based on a servo synch mark (SSM) that conflicts with self-clocking encoding algorithms.
2. Description of the Prior Art and Related Information
Computer systems rely on rotating media storage devices (RMSDs), which often employ a moveable head actuator to frequently access large amounts of data stored on the media. One example of an RMSD is a hard disk drive. A conventional hard disk drive has a head disk assembly (“HDA”) including at least one magnetic disk (“disk”), a spindle motor for rapidly rotating the disk, and a head stack assembly (“HSA”) that includes a head gimbal assembly (HGA) with a moveable transducer head for reading and writing data. The HSA forms part of a servo control system that positions the transducer head over a particular track on the disk to read or write information from and to that track, respectively.
With reference to FIG. 1, FIG. 1 shows an example of a prior art disk 10 having a plurality of concentric tracks 12. Each surface of each disk 10 conventionally contains a plurality of concentric data tracks 12 angularly divided into a plurality of data sectors 15. In addition, special servo information is provided on each disk to determine the position of the moveable transducer head.
The most popular form of servo is called “embedded servo” wherein the servo information is written in a plurality of servo wedges 14a, 14b, etc. that are angularly spaced from one another and are interspersed between data sectors 15 around each track of each disk.
Each servo wedge 14 typically includes a phase lock loop (PLL) field 20, a servo synch mark (SSM) field 22, a track identification (TKID) field 24, a wedge ID field 26 having a binary encoded wedge ID number to identify the wedge, and a group of servo bursts (e.g. ABCD) 26 (e.g. an alternating pattern of magnetic transitions) which the servo control system samples to align the moveable transducer head with or relative to a particular track.
Typically, the servo control system moves the transducer head toward a desired track during a coarse “seek” mode using the TKID field as a control input. However, in processing information, it is necessary to ensure consistency in the detection of bits composing a block of bits. One common approach directed to ensuring such consistency employs multiple stored fields including a phase lock loop (PLL) field 20 to facilitate bit synchronization and a synch field to facilitate block synchronization. The synch mark field facilitates block synchronization by holding a special marker that is detected to “frame” data, i.e., to identify a boundary of a block. In contemporary hard disk drives employing embedded servos, it is well known to provide framing of servo data via a servo synch mark (SSM) field 22.
Generally, in hard disk drives, a servo synchronization signal based on the head reading a servo synchronization mark (SSM) results in a read/write channel of the disk drive establishing a precise timing reference point for read/write operations.
Once the transducer head is generally over the desired track, the servo control system uses the servo bursts (e.g. ABCD) 28 to keep the transducer head over that track in a fine “track follow” mode. During track following mode, the moveable transducer head repeatedly reads the wedge ID field 26 of each successive servo wedge 14 to obtain the binary encoded wedge ID number that identifies each wedge of the track. In this way, the servo control system continuously knows where the moveable head is relative to the disk.
As previously discussed, a servo synchronization signal based on the head reading a servo synchronization mark (SSM) 22 typically causes a read/write channel of a disk drive to establish a precise timing reference point for any read/write operations. Thus, it is important that the servo synchronization signal be robust and timely. To that end, the SSM pattern should be unique such that it will not be identified in other areas of the servo wedge. Particularly, it is important that the SSM pattern not be mistakenly identified as the TKID field 24, the wedge ID field 26, the servo bursts (e.g. ABCD) 28, etc.
Typically in most disk drives, the SSM 22, the TKID 24, and the wedge ID 26 are all recorded and encoded in accordance with a self-clocking encoding algorithm on the disk. Self-clocking encoding algorithms provide an encoding method in which data as well as clocking is integrated into one encoded pattern. One of the most commonly used types of self-clocking encoding algorithms is Manchester encoding.
Turning now to FIG. 2, FIG. 2 illustrates an example 2 of Manchester encoding. As can be seen in FIG. 2, Manchester encoding defines the time required to define a bit into two cycles. In one example 3, an up-going pulse, defines a data value of “1” by having a first cycle of positive bits (1, 1) followed by a down-cycle of zero bits (0,0). A series of all data “1's” in accordance with Manchester encoding can be seen in pattern 5. The dashed lines represent the read-back signal generated by the head of the disk drive as it reads the encoded pattern. Conversely, as shown in example 4, a data value of “0” in Manchester encoding can be defined as a down-going pulse having a first cycle of two zero bits (0,0) followed by an up-cycle of positive bits (1, 1). A series of all data “0's” in accordance with Manchester encoding can be seen in pattern 6. The dashed lines represent the read-back signal generated by the head of the disk drive as it reads the encoded pattern. Further, an example pattern of data bits, e.g. 1, 0, 1, 1, 1 in accordance with Manchester encoding, as recorded on the disk, can be seen as example pattern 7. Again, the dashed lines represent the read-back signal generated by the head of the disk drive as it reads the encoded pattern. It should be appreciated that this is one example of Manchester encoding and other variations are possible.
In current disk drives, this type of self-clocking Manchester encoding is typically used in encoding the SSM 22, the TKID 24, and the wedge ID 26. Because the SSM, the TKID, and the wedge ID all utilize the same self-clocking Manchester encoding algorithm, they are more likely to be misrecognized as one another.
However, if the SSM pattern is mistakenly identified in one of the other areas of the servo wedge, read/write operations may be compromised resulting in the wrong data being read, or, data being written to areas of the disk that is not supposed to be. Unfortunately, due to the fact that all of these various servo wedge components utilize the same self-clocking Manchester encoding algorithm, there is a greater likelihood that the SSM pattern will be misrecognized in other areas of the servo wedge.